Although the U.S. Supreme Court has already affirmed the constitutionality of the federal health care law, Missouri voters will get to cast ballots in November on another of the law’s key components — insurance exchanges.

Proposition E on the ballot would prohibit Gov. Jay Nixon or any state agency from setting up a health insurance exchange without the approval of the legislature or the state’s voters.

A health insurance exchange is a web-based marketplace for consumers to compare insurance plans. The exchanges are a central part of the federal law, passed in 2009, because they are intended to help individuals and small businesses purchase insurance at an affordable rate to comply with the part of the law that mandates most people own health insurance.

Nixon has said that his administration would not move to set up an insurance exchange by decree. But conservatives say they want to make sure the Democratic governor is barred from doing so if he wins a second term, especially after the U.S. Supreme Court upheld the law in a 5-4 decision on June 28.

The Supreme Court’s ruling, however, makes it nearly certain that the requirement for health care exchanges will take effect unless congressional Republicans repeal the law. If the state does not set up an exchange, the federal government will do it.

That has groups who supported the law, such as Missouri Health Care for All, saying that Missouri should simply prepare its own exchange.

“Prop E is really just an attempt to continue to use health care reform for political gain because there’s really going to be no practical effect because health reform, the Affordable Care Act, is the law of the land,” said MHCA President Susan Talve, a St. Louis rabbi, in a telephone interview.